I fully accept your points.
IS are murdering raping scum, certainly, but they are still human beings. Unfortunately, they are not reasonable human beings.
Do you really see a diplomatic way out of this? Partial capitulation may help, allow IS to hold the territory that they now hold, with a meaningful threat that their will be boots on the ground should they try and enlarge it. There would also need to be agreement to let those people leave their territory that wished to.
If I thought for a moment that the above would work, I would reluctantly endorse it, however, I don't think that that, or any variant thereof, would work.
IS is not going to lay down its arms other than by being confronted with overwhelming force.
Edited to add:
Iran seems to be coming out of the cold at the moment, albeit slowly.